KIEV, July 14 (Reuters) - Ukrainian President Petro Poroshenko said on Monday Russian military staff officers were fighting against Ukrainian forces alongside separatist rebels and a new Russian missile system was being used on the separatist side.

"In the past three days a new Russian missile system has been used against the armed forces of Ukraine," he told security chiefs.

"Information has also been confirmed that Russian staff officers are taking part in military operations against Ukrainian forces," he said, according to his website.